Currently running a trial with Release 7.2.0_GA_2669.RHEL6_64_20120410002117 CentOS6_64 NETWORK edition.
Came across same issue where deleted accounts were still being counted against the License (in this case 50)

Could not provision with either admin console or CLI (zmprov)

As per Bug 24009, ran as user zimbra zmprov flushcache license
This updated the admin console count of used accounts to the correct value, and allowed provisioning via both admin console and CLI.